Trevor paints 1/6 of the fence surrounding his farm each day. how many days will it take him to paint 3/4 of the fence?

To do this you have to divide 3/4 by 1/6.  To do this multiply 3/4 by the reciprocal of 1/6.  The reciprocal of 1/6 is 6/1.  So you have to multiply 3/4 and 6/1.  This will give you 18/4.  You now simplify it to 4 2/4, which can be simplified even more to 4 1/2.  It will take Trevor 4 1/2 days to paint 3/4 of the fence.  
BTW- In case you are wondering how long it'll would take him to paint the entire fence, it would be 6 days.